The story's protagonist is a boy named Conradin who is slowly dying of an unspecified disease. Conradin lives with his older cousin, a woman named Mrs. de Ropp whom he despises. Unknown to Mrs. de Ropp, Conradin has a pet ferret which he keeps in a tool shed in the garden. Conradin names the ferret Sredni Vashtar and begins to develop his own religion in which Sredni Vashtar is a god. When Mrs. de Ropp decides that she does not want Conradin to go to the tool shed anymore, Conradin repeatedly prays to Sredni Vashtar, asking the animal deity to do just one thing for him.